Prior art schemes to obtain constant power include the common voltage regulator. Such regulators, particularly the series pass regulator, are very inefficient since they must dissipate power equal to all of the load current times the difference between the unregulated and the regulated voltages. A switching regulator is more efficient than the series pass regulator but is usually more complicated and requires many more components. Other schemes to obtain regulated power involve complex analog building blocks such as multipliers and dividers. As the number and complexity of components increases, so does the cost.
The preferred embodiment of the present invention employs one quad comparator, 11 resistors and capacitors, and one zener diode and has an efficiency of approximately 90%. The circuit is designed to provide an output voltage having a root mean square (RMS) value regulated as a function of the duty cycle of an A.C. component and the magnitude of the unregulated D.C. component. The duty cycle is inversely proportional to the square of the D.C. component. The power regulator of the present invention provides a constant RMS voltage whose value is within 1% of nominal for a D.C. input voltage variation of approximately .+-.14%.
To maintain uniform print contrast, thin film thermal printer resistors must be energized to approximately the same temperature each time they are energized. The same temperature must be attainable even where the power source may not be constant, such as a battery. These resistors typically have a characteristic thermal time constant, i.e. a finite response time to heat up to a temperature proportional to the magnitude of D.C. voltage applied. Under constant use at lower D.C. voltage levels or if too much power is applied even for shorter time periods, these resistors tend to become thermally stressed, unreliable and ultimately crack over a relatively short time in use. These difficulties are overcome by employing the principles of the present invention. Where the period of the A.C. component of the output power waveform is less than the thermal time constant for the thin film resistor, the instantaneous voltage pulses will be averaged out to appear as a constant energy source without producing the thermal stresses in the resistor produced when a step function of purely D.C. voltage is applied. Thus, the useful life of each resistor will be extended yet will develop a constant temperature when energized for printing purposes.